This chapter focuses on a biological process for hydrogen generation that depends on the environmentally benign use of biomass and solar energy. A group of anoxygenic photosynthetic bacteria known as purple nonsulfur bacteria (PNSB) produce large amounts of hydrogen under normal growth conditions by using nitrogenases as opposed to hydrogenases. However, hydrogen production by this route tends to be short-lived because of the extreme oxygen sensitivity of hydrogenases and nitrogenases. The chapter reviews the fundamental biology of nitrogenase-catalyzed hydrogen production by PNSB. Researchers found that photohydrogen production was inhibited by nitrogen gas. There are numerous strategies for strain development that can be expected to lead to improvements and stabilization of the hydrogen production process. In addition to their practical usefulness, the application of such strategies will lead to an improved understanding of the hydrogen production process as it operates in the context of whole cells. The fundamental unit of peripheral light-harvesting systems, also known as light harvesting 2, consists of two other types of α and β polypeptides. Two metabolic processes that consume large quantities of reductant and thus have the potential to divert electrons away from nitrogenase-catalyzed hydrogen production by whole cells of PNSB are poly-hydroxyalkanoate (PHA) synthesis and carbon dioxide fixation. In many ways studies of nitrogenase-catalyzed hydrogen production by anoxygenic phototrophic bacteria are still in their infancy.

Nitrogenase-catalyzed hydrogen production by PNSB. PNSB can generate the electrons needed for hydrogen production by oxidizing organic compounds and selected inorganic compounds. They generate ATP by cyclic photophosphorylation. Protons derive from water or are generated along with electrons when organic compounds are oxidized. The theoretical stoichiometries for nitrogenase reactions for hydrogen production in the presence and absence of nitrogen gas are indicated. Reprinted from Rey et al. (2007) with permission of the publisher.

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Figure 1.

Nitrogenase-catalyzed hydrogen production by PNSB. PNSB can generate the electrons needed for hydrogen production by oxidizing organic compounds and selected inorganic compounds. They generate ATP by cyclic photophosphorylation. Protons derive from water or are generated along with electrons when organic compounds are oxidized. The theoretical stoichiometries for nitrogenase reactions for hydrogen production in the presence and absence of nitrogen gas are indicated. Reprinted from Rey et al. (2007) with permission of the publisher.

The nitrogenase reaction. Ferredoxins (Fd) and flavodoxins (Fld) transfer electrons generated from the oxidation of organic compounds during metabolism to the Fe protein to initiate a reaction cycle. Reprinted from Howard and Rees (1994) with permission from the Annual Review of Biochemistry.

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Figure 2.

The nitrogenase reaction. Ferredoxins (Fd) and flavodoxins (Fld) transfer electrons generated from the oxidation of organic compounds during metabolism to the Fe protein to initiate a reaction cycle. Reprinted from Howard and Rees (1994) with permission from the Annual Review of Biochemistry.

The photosynthetic apparatus of the PNSB, depicting ATP generation by cyclic photophosphorylation. Light energy absorbed by the peripheral light antenna (LH2) is transferred to the core light antenna (LH1) and then the reaction center (RC). These components can also absorb photons directly at the wavelengths indicated. The gray arrows indicate energy transfer reactions, and the black arrows indicate electron or proton transfer reactions. Ubiquinone molecules (Q), which are mobile in the membrane, accept two protons from the inside of membrane vesicles along with energized electrons from the reaction center. Cytochrome b/c1 catalyzes electron transfer between ubiquinol (QH2) and the mobile electron carrier, cytochrome c (cyt c ). Electron transfer to cytochrome c is coupled to the translocation of protons across the membrane to create a proton gradient. The cyclic flow of electrons back to the reaction center is completed by cytochrome c. ATP synthase uses the proton gradient to generate ATP. Adapted from Cogdell et al. (2006).

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Figure 4.

The photosynthetic apparatus of the PNSB, depicting ATP generation by cyclic photophosphorylation. Light energy absorbed by the peripheral light antenna (LH2) is transferred to the core light antenna (LH1) and then the reaction center (RC). These components can also absorb photons directly at the wavelengths indicated. The gray arrows indicate energy transfer reactions, and the black arrows indicate electron or proton transfer reactions. Ubiquinone molecules (Q), which are mobile in the membrane, accept two protons from the inside of membrane vesicles along with energized electrons from the reaction center. Cytochrome b/c1 catalyzes electron transfer between ubiquinol (QH2) and the mobile electron carrier, cytochrome c (cyt c ). Electron transfer to cytochrome c is coupled to the translocation of protons across the membrane to create a proton gradient. The cyclic flow of electrons back to the reaction center is completed by cytochrome c. ATP synthase uses the proton gradient to generate ATP. Adapted from Cogdell et al. (2006).

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